An artificial sphincter may be utilized in any number of applications within a patient's body where it is desirable to vary the size of an orifice or organ. Depending upon the application, artificial sphincters may take the form of a flexible, substantially non-extensible band containing an expandable section that is capable of retaining fluids. The expandable section would be capable of expanding or contracting depending upon the volume of fluid contained therein. One particular example of an artificial sphincter is an adjustable gastric banding device, such as described in U.S. Pat. Nos. 4,592,339, 5,226,429, 6,102,922, and 5,449,368, the disclosure of each being hereby incorporated by reference. Since the early 1980s, adjustable gastric bands have provided an effective alternative to gastric bypass and other irreversible surgical weight loss treatments for the morbidly obese.
The gastric band is wrapped around an upper portion of the patient's stomach just inferior to the esophago-gastric junction, forming a stoma that restricts food passing from an upper portion to a lower portion of the stomach. When the stoma is of the appropriate size, food held in the upper portion of the stomach provides a feeling of fullness that discourages overeating. However, initial maladjustment or a change in the stomach over time may lead to a stoma of an inappropriate size, warranting an adjustment of the gastric band. Otherwise, the patient may suffer vomiting attacks and discomfort when the stoma is too small to reasonably pass food. At the other extreme, the stoma may be too large and thus fail to slow food moving from the upper portion of the stomach, defeating the purpose altogether for the gastric band. Thus, different degrees of constriction are desired, and adjustment is required over time as the patient's body adapts to the constriction.
In addition to a latched position to set the outer diameter of the gastric band, adjustability of gastric bands is generally achieved with an inwardly directed inflatable balloon, similar to a blood pressure cuff, into which fluid, such as saline, is injected through a fluid injection port to achieve a desired diameter. Since adjustable gastric bands may remain in the patient for long periods of time, the fluid injection port is typically installed subcutaneously to avoid infection, for instance in front of the sternum or over the fascia covering one of the oblique muscles. Adjusting the amount of fluid in the adjustable gastric band is achieved by inserting a Huber tip needle through the skin into a silicon septum of the injection port. Once the needle is removed, the septum seals against the hole by virtue of compressive load generated by the septum. A flexible catheter communicates between the injection port and the adjustable gastric band.
While the injection port has been successfully used to adjust gastric bands, it would be desirable to make adjustments noninvasively. Insertion of the Huber tip syringe is typically done by a surgeon, which may be inconvenient, painful, or expensive for the patient. In addition, a skin infection may occur at the site of the insertion of the syringe. Consequently, it would be desirable to remotely control an adjustable gastric band.
In an afore-mentioned co-pending application entitled “PIEZO ELECTRICALLY DRIVEN BELLOWS INFUSER FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Ser. No. 10/857762, an advantageous infuser containing no ferromagnetic materials provides an accurately bi-directionally controllable volume of fluid to a closed gastric band. The infuser has a titanium bellows accumulator, which may be collapsed or extended to positively displace fluid accumulated therein, thereby serving as both a reversible pump and reservoir. Thereby a bi-directional pump that is practically immune to external magnetic fields is achieved, unlike previously known implants that contained a metal bellows for controllable dispensing of a liquid drug, such as described in U.S. Pat. No. 4,581,018. Thereby, such an implanted device may undergo Magnetic Resonance Imaging (MRI) without damage to the device or patient.
Accurate delivery of fluid from a bellows accumulator benefits from a feedback control system and as a means for determining bellows position relative to its housing. In the U.S. Pat. No. 4,581,018 patent, position feedback was provided by a rotary encoder connected to the output shaft of an electrical motor used to rotate the cylindrical metal bellows. In this instance, the rotations of the motor are determined by counting incremental marks on an optical disk of the encoder for the purpose of dispensing at a uniform rate.
However, in creating a device that may be required to pump in both directions with long intervals therebetween, it is believed that counting rotations of a rotary actuator in some instances may inaccurately reflect the volume in the bellows accumulator. For example, the current volume of the reservoir is not directly sensed, and thus integrating a rate of dispensing to calculate a change in volume still suffers if the starting point is not known or if fluid is transferred inadvertently due to leakage or other factors.
In other infuser devices, sensing fluid pressure within the bellows accumulator has been used as an indirect measurement of volume, relying upon a fixed relationship in pressure and volume since the bellows accumulator is collapsed based on a gauge pressure exerted thereon by a propellant within an infuser device housing. However, such pressure sensing assumes that the fluid pressure is not varied by pressure external to the infuser device, such as would be expected in a closed artificial sphincter system. Specifically, the amount of back pressure would vary somewhat unpredictability. Examples of such pressure-based sensing include U.S. Pat. No. 5,507,737 (pressure gauge), U.S. Pat. No. 5,974,873 (strain gauge), and U.S. Pat. No. 6,315,769 (spring and pressure sensitive resistor).
Recently, it has been recognized as desirable to sense remaining fluid volume in a drug dispensing infuser device in order to determine when refilling is necessary. To that end, U.S. Pat. No. 6,542,350 discloses forming a variable capacitance between the bellows accumulator and the infuser device housing to sense volume. Similarly, U.S. Pat. No. 6,482,177 discloses forming a variable inductance between the bellows accumulator and the infuser device housing to sense volume. In both instances, using the sensed volume for such purposes was not suggested other than relaying a value by telemetry for display to a human operator. This is understandable in that these drug dispensing applications meter small amounts of a drug without significant variations in external backpressure to the infuser. Continuous volume sensing was not addressed. Power consumption for volume sensing would create an undesirable increase in battery size. In addition, accuracy of the variable capacitors or inductors may have been insufficient for these purposes, especially as the portions of the variable capacitor or inductor move away from one another in the presence of electromagnetic interference.
Consequently, a significant need exists for sensing a position of an implanted bellows accumulator representing a fluid volume for closed loop control of an implanted artificial sphincter.